Systems for thermal development of flexographic printing plates are well known in the art. A typical system for thermal development of flexographic printing plates comprises a plate processor that accepts a previously formed and imagewise actinic radiation exposed flexible sheet. The flexible sheet typically has a base layer and one or more adjacent layers of a radiation hardenable elastomer material (curable layer). Optionally, an infrared sensitive layer may be present on top of the radiation-curable layer.
U.S. Pat. No. 5,175,072 to Martens, the subject matter of which is herein incorporated by reference in its entirety, describes a method for forming, irradiating, and thermally developing a flexographic sheet to produce a flexographic printing plate. Thermal development plate processors are further described in U.S. Pat. No. 5,279,697 to Peterson et al., and in WO 01/18604 to Johnson et al., the subject matter of each of which is herein incorporated by reference in its entirety. The plate processor described by Johnson et al. is similar to the plate processor described by Peterson et al.
Prior to using the plate processor to thermally develop the flexible sheet to form the flexographic printing plate, the flexographic printing plate is subjected to an imaging step. In a typical imaging step, a template or mask is first placed over the radiation-curable layer, which is imagewise exposed to actinic radiation to harden (cure or crosslink) a portion of the radiation curable layer. Alternatively, a laser ablatable layer may be ablated by a laser to create an in situ mask on the radiation-curable layer. A portion of the radiation-curable layer is then cured by actinic radiation through the lower surface of the base to form a cured “floor.” Next, the radiation-curable layer is imagewise exposed from the opposite surface to cure the desired portions of the plate. The radiation-curable layer after curing consists of cured portions and uncured portions. At this point, the flexible sheet may be developed in the thermal plate processor to remove uncured portions of the printing plate, revealing the relief image on the printing plate.
The development of the relief structure formed during the imaging step is accomplished by removal of the uncured portions of the radiation-curable layer. In the thermal development process, the radiation-curable layer is contacted with a heated web of absorbent material. The heat in the absorbent web is transferred to the radiation curable layer upon contact, and the temperature of the radiation-curable layer is raised to a temperature sufficient to enable the uncured portions of the radiation-curable layer to soften or liquefy and be absorbed into the absorbent web. While still in the heated condition, the absorbent sheet material is separated from the cured radiation-curable layer in contact with the support layer to reveal the relief structure. After cooling to room temperature, the resulting flexographic printing plate can be mounted on a printing plate cylinder.
After the developing step, conventional post-treatment steps including post-exposure and detackification can be utilized.
The post-exposure step in the plate-making process is a step of irradiating the resin cured product after the development with actinic radiation, aiming mainly at accelerating the mechanical strength and removing the surface tackiness. As an example of the post-exposure step, an actinic radiation source having a wavelength distribution in a range of 300 nm or (for example, high pressure mercury vapor lamp, ultra-high pressure mercury vapor lamp, ultraviolet fluorescent lighting, carbon arc lamp, xenon lamp or the like) and an actinic radiation source having a wavelength distribution in the range of from 200 to 300 nm (for example, low pressure mercury vapor lamp, germicidal lamp, deuterium lamp or the like) are used in combination.
Surface tackiness of the printing plate can cause various problems in the printing process, including poor workability during the process of attaching the plate to and detaching the plate from the plate cylinder, adhesion of plates to each other or attachment of dust to the plates during the storing of the plates, adhesion of paper powder to the plate and paper-picking during the printing, etc. In particular, when paper powder adhesion and the paper-picking result from the tackiness of the plate during the printing, it is necessary to discontinue the printing and clean the plate, and the working efficiency of the plate is markedly decreased. One example of a surface-treating solution that can reduce surface tackiness is described in U.S. Pat. No. 6,403,284 to Yoshida, and comprises a hydrogen-abstracting agent contained in an aqueous surface active agent solution. Various after treatment solutions can also be utilized, such as those described in U.S. Pat. No. 5,124,237 to Littmann et al. and U.S. Pat. No. 4,725,528 to Koch et al.
Other surface treatments for flexographic printing are commonly applied manually after processing, and are used to aid in printing performance and longevity. Examples include Armor All® and other silicone emulsions. Surface treatments may be desired for various reasons including to increase resistance to ozone or UV degradation, to change surface properties that may affect ink acceptance or release, or to affect other changes in plate properties or processing characteristics that improve plate performance.
When printing plates are exposed to the air in a standard printing room, particularly in the warmer months of the year, ozone present in the air can cause cracking of the relief surface. Ozone cracking is manifested generally by the initial appearance of short, deep cracks perpendicular to the direction of curvature of the plate, i.e., across the cylinder. These cracks may start on the printing surface, but when cracking is severe, the cracks go over the edge of the relief surface onto the floor of the printing plate. The thicker the plate, the more readily it cracks because of the grater stress caused by wrapping the plate around the printing cylinder. Severe cracks can damage the plate sufficiently to render it useless for printing. In order to prevent plates from cracking, U.S. Pat. No. 4,452,879 to Fickes et al. suggests the use of a dithiocarbamate solution that is applied to a dried relief surface of the printing plate.
High ozone concentrations can also cause difficulties when flexible packaging materials such as paper, cardboard, and plastic films are printed on. In order to obtain sufficient adhesion of the printing ink to the plastic film, the films may be pretreated with electrical spark discharge, which produces significant quantities of ozone. Because this pretreatment is most effective when applied immediately before the printing process, it is unavoidable that the flexographic printing plates are exposed to ozone. In this instance, ozone protection may be provided by applying a treating material such as a liquid polyether, to the printing plate, as described in U.S. Pat. No. 4,680,251 to Schober. The application of the treating material onto the printing plate may be carried out manually by using a cotton swab, sponge or brush, or by dipping the printing plate into a bath. The polyether can also be applied mechanically by spraying or rinsing.
After the printing plate has been exposed imagewise and developed, non-image areas may be desensitized by adding a thin, tightly adhering film of a hydrophilic colloid, such as a gum. This hydrophilic film (desensitizer gum) acts as a barrier to prevent ink from sensitizing the non-image areas of the plate. The desensitizer gum is typically applied with a cotton pad and a rubber squeegee used to remove excess gum and ready the plate for printing. The use of desensitizing gums is discussed, for example, in U.S. Pat. No. 3,745,028 to Rauner with respect to lithographic printing plates.
As demonstrated by the above discussion, a variety of surface treatment compositions, may be applied to relief image printing plates to improve properties of the printing plate. However, many of these surface treatment compositions are applied manually, resulting in uneven application of the surface treatment composition. Furthermore, most, if not all of the application methods require an additional step, thus requiring additional time and an additional apparatus.
To that end, further improvements are needed to more efficiently and uniformly apply surface treatment compositions to relief image printing plates to improve the quality of such plates. The inventors have discovered that a thermal processing system used to develop the relief image of the photosensitive printing element can advantageously be used to apply a variety of surface treatment compositions uniformly and automatically to the surface of relief image printing plates.